Conventional chain saws are equipped with a guide bar having a rounded nose portion at its outer end. The nose portion may be provided with an idler sprocket, or roller nose or the chain may run on a sprocketless but generally hardened nose portion. A groove extends along the edge of the bar, and drive links of the saw chain run in the groove. The conventional saw chain itself consists of centered drive links pivotally joined by rivets to cutter links, balancing links and tie links, with cutter links positioned alternatively on opposite sides of the chain. A balancing link, wich may be identical with the tie links is positioned opposite each cutter link. The drive links run in the groove in the guide bar, and in turn are driven by a sprocket or ring mounted on the power take off shaft of a gasoline engine, or other power unit. In order to reduce the overall weight of the chain saw, two stroke engines operating at speeds up to fourteen thousand (14,000) RPM are used to drive the chain. A centrifugal clutch is conventionally included in the saw to disengage the drive to the chain below a predetermined speed, so that the saw when idling has a stationary chain. The chain is generally intended to be run at a lineal speed of 3000 to 4000 feed per minute, and with approximately eight cutters per foot, each designed to cut approximately 25 thousands of an inch deep. Thus extremely high cutting rates are obtained with a light weight portable tool.
In normal use, cuts are made with a chain saw by placing the lower edge of the guide bar on the material to be cut, and as the chain travels around the bar, each tooth contacts the material and removes a chip or chips with each pass through the saw kerf. The shape and size of the chips is dependent on many factors including the shape of the cutter, the height of the depth gauge, the grain of the material, and many other factors.
Other types of cuts may also be made with a chain saw. Frequently during limbing of a felled tree, the upper edge of the guide bar is applied from beneath a limb and the guide bar is urged upwardly to sever the limb. Boring cuts are also made by pressing the nose of the guide bar against the material being cut.
It will be appreciated that the speed and power of a modern light weight chain saw, also creates a potential hazard due to accidental or careless operation of this tool. What is not widely recognized either by chain saw manufacturers or users, is that seemingly small differences in the design of the cutting attachment can turn a tool referred to as a "motorized widowmaker" into a safe, dependable, implement.
Aside from careless handling, which can always be fatal, the next greatest hazard is "kickback".
A kickback is a phenomena which occurs when a cutter of the chain rounding the nose of the bar strikes an object and the momentum of the chain is converted into a force, creating a couple around the handle of the chain saw, causing the chain saw to rotate instead of the chain to translate. This is analogous to the type of couple created when a load is hitched too high on a farm tractor, causing the tractor to overturn backward on top of its driver, instead of drawing the load.
Kickbacks can occur during limbing, buried nose cutting and boring operations, and result too frequenlty in the moving saw chain coming in contact with the head or upper body of the operator. Grievous wounds are caused which are frequently fatal, and if not, require sophisticated surgical intervention. Plastic surgeons are generally required to repair such wounds.
The present invention is directed to the improvement in the safety, versatility and efficiency of cutting attachments for chain saws. These improvements overcome the defects of known prior art cutting attachments both in cutting efficiency, versatility, safety, and from the users standpoint, longevity, both of himself and his machine.
The chain saw industry has devoted much time and effort to solve the problem of kickbacks. One such attempt is the chain brake. This is a device which is intended to stop the chain if a kickback occurs, and hopefully before the chain strikes the operator. Chain brakes do not work! Bloodless corpses have been found in the woods beside an idling chain saw, and the chain brake was on!
Other attempts to solve the kickback problem have included sprocket guide bar noses, specially shaped depth gauges, safety links, asymmetrical guide bars, the so called "banana bar," and others. Possibly the greatest prior art step in the right direction is the work of Curtis L. Graverson of Milwakie, Ore., the inventor of the low profile saw chain, for which he was granted U.S. Pat. No. 3,929,049 on Dec. 30, 1975. Graverson was concerned with the problem of kickback as may be seen for example at column 2 line 17 et seq of the above patent.
Because of the risk of kickback, boring operations are difficult and hazardous. It has been found that the portion of the saw chain entering the cut, during boring operations, becomes slack and the cutters tend to tilt upwardly on the guide bar, resulting in excessive cut and vibration. "Lifting" of the chain off the bar as it is carried around the nose by means of an idler sprocket imposes severe wedging loads on both the drive links and the rivets, stretching and loosening the chain, while lowering of the chain back on to the bar, after passing around the nose, causes hammering, impactation, wear, and vibration. Further, the cutters are poorly supported in a rlatively unstable condition when the chain is lifted clear of the bar as contact is only made via the sprocket teeth inserted between two pivoting rivets and the unbalanced drive links in the groove of the bar.
The contact with the drive links is generally unbalanced because of the sawdust cleaning feature provided therein, and these factors further contribute to the hazardous kickback condition. Furthermore, the idler sprocket teeth tend to jam the chain with wood chips propelled to it by the chip cleaning device on the drive link. This forces the operator to work with a loose chain.
Secondary (grease gun) lubrication of the sprocket at the nose portion of the bar is relatively difficult and further contributes to the wear problem. It should be appreciated that uninterrupted and effective lubrication of the saw chain as it slides around the guide bar is by far the major contributor to control chain wear and kickback.